The Standard Eye and Other Inconsistencies

Just when we
were feeling comfortable
with the
CIE photopic response
curve of 1924, we discover
that this standard
has been completely incorrect
for nearly ninety
years.

Before you enter into a state of complete
despair and consider another career,
we will try to determine what effect this
error will have on the entertainment lighting
industry. What changes will you need to
make in your current operating method to
accommodate this significant revelation?

EYE CELLS
First, we have to present some of the basic
concepts of vision and the perception
of color. You may have heard this before
but, for completeness, a review is necessary.

The eye is composed of four types of
photoreceptor cells, cells that respond to
electromagnetic radiation over a very specific
range of wavelengths—the visible
spectrum. The first are rod cells, which
are not color sensitive and respond at very
low light levels. (Think “night vision.”) This
type of vision is called scotopic. We need
go no further with this as the light levels
we are concerned with are much greater
than for this type of vision.

The second group is the cone cells and
these operate throughout the broad range
of general vision. This type of vision is
called photopic. There are three types of
cone cells and each responds to specific a
group of wavelengths that are roughly red,
blue and green. Because of this fact, only
three numerical components corresponding
to these three receptors are necessary
to describe the colors we see. In other
words, we were blessed with a tri-stimulus
color capability before Newton’s great
color theory. During photopic vision, the
eye is also capable of adapting to a wide
range of illuminance levels, light levels that
go from moonlight to sunlight. An amazing
device.

PAINTING
Color systems are idealized as being
composed of two sets of information. First,
we have the luminance information, which
is related to the eye’s perception and sensitivity
to brightness only. (Think of it as a
monochrome signal.)

We then paint this signal with the color
information to restore the basic color image.
It is the way that television’s color signal
is generated. It happened this way as a
result of a curious, but convenient, set of
circumstances.

THE EARLY DAYS
When commercial television started,
the camera contained only a single sensor,
an electron tube with a photo-electric
surface sensitive to radiant energy. As fate
would have it, the sensitivity of this device
to the spectrum was not the same as that
of the eye. It did not respond to the visible
spectrum in the same manner as an eye.

Fig. 1: Luminosity Function (Photopic)

A great deal of experimentation was
done to approximate the eye’s spectral
response. For example, a common early
imaging device was extremely sensitive to
the infrared portion of the spectrum. Filters
were placed in the camera to limit not
only the infrared region, but the ultraviolet
end of the spectrum as well.

To complete this task, test groups of
people were assembled and asked to evaluate
pick-up devices with various spectral
responses as well different transfer functions
(“gamma”) and to judge them as to
their ability to produce “perfect reproduction.”
They viewed the real scene and then
the scene viewed on the display device of
the time, the cathode ray tube. The early
image orthicon camera was the result.

Of course, without the color information—
only luminance—it could hardly be
“perfect reproduction.”

In addition, because of the impossible
task of obtaining a reasonable signal-tonoise
ratio, the end result was heavily compromised
to mask this defect.

When television went to color, one of
the early issues was that all existing blackand-
white sets needed to be able to receive
and view the new color signal. As a result,
the portion of the new color signal that the
older sets would respond to was proportionate
to the luminance property of the
image. Meanwhile, the color information
was separate, waiting to be painted on this
signal when viewed on a color television
set.

RETURNING TO 1924
Let us return to 1924. The CIE or Commission
Internationale de L’éclairage was
established in 1913 as the international
authority on light, illumination, color and
color spaces. In 1924, it defined the “standard
photopic observer” and published
the famous curve that shows the standard
eye’s response to the wavelengths of the
visible spectrum.

The manner in which the “color” information
was removed so that the test subjects
were only comparing the luminosity
of individual narrow bands of wavelengths
to the luminosity of the wavelength of 555
nanometers (the green wavelength where
the eye is most sensitive) is quite interesting
and perhaps was the direct cause of
the dilemma of today.

TODAY
What has happened
in further studies over
the years is that, as testing
techniques were
refined and the sample
has increased, the standard
observer has been
redefined. What the scientists
realized was that
the sensitivity of the
standard eye in the blue
end of the spectrum was
actually quite a bit more
sensitive than had been
determined in 1924.

In Fig. 1, the blue area
represents the differences
that resulted from
this later research. Just so
you know, the most current
group of researchers
chose a group of 40 subjects,
ages 18–48, which
includes five females.
I’m not sure how this compares with the
group of 1924.

For illustration, if we compare the two
results at the wavelength of 450 nanometers
(a deep blue), we see that even if the
response of the eye at this wavelength is
low relative to the sensitivity maximum
value at 555 nm, the ratio of the new-toold
value at 450 nm is easily two times, a
significant departure.

So what?

As more and more sources with discontinuous
spectrum were used—fluorescents
and now, LEDs—lighting people
were aware and confused by the fact that
blues appeared much brighter to the eye
than their light meters indicated. Light meters
are monochromatic devices whose
sensitivity is based upon (you guessed it)
the Luminosity Function.

To rectify this part of the dilemma is
quite simple, but encumbered with an impractical
reality. In order to correct this inadequacy,
every incident light meter in existence
will have to be thrown away and a
new, more accurate meter purchased (after
some redesign, of course).

Attempting to alter the
sensitivity of the meter to
match the new, corrected
CIE curve, would probably
invite the development of
a new, but very questionable
cottage industry.

WE’RE OKAY IN TV
LAND
From a very rigorous
viewpoint, we should also
change the algorithms
that are applied in camera
circuitry to obtain the luminance
signal. But these
formulas have not been
appreciably altered since
the beginnings of color
TV. They would have to be
tested with groups similar
to the 40 subjects under
controlled laboratory conditions
to detect the departure
from the ideal.

I think we can safely say that the luminance
signal of a camera is close enough
for “perfect reproduction.”

We need only plan for a new, improved
light meter.

Bill Klages would like to extend an invitation
to all the lighting people out there
to give him your thoughts at billklages@roadrunner.com.